Various tools for colour calibration have been used for a long time in connection with traditional photography with light sensitive film. It is an advantage of such colour calibration tools that they facilitate the selection of film, the adjustment of camera and illumination, the selection of colour filters, if any, and the selection of development conditions and reproduction settings, so that the best possible end result can be achieved. So-called colour calibration cards, i.e. strips, sheets or cards of paper or other suitable material having pre-printed calibration fields of selected colour and grey tones, is one type of such previously known tools for colour calibration. By placing such a colour calibration card adjacent to the intended subject, information is thus obtained which facilitates the photography, developing and/or later reproduction, thereby providing a better end result. As a rule, these previously known colour calibration cards have calibration fields which are primary colours, which provide grey scales, which have been selected to resemble different colours in the nature, such as colours of sky, foliage, water, and the like, or which represent different skin tones. The previously known colour calibration cards for traditional photography are thus to a great extent based on the hues of the expected subjects of the photography.
Colour calibration cards which have been adapted specially for photography with digital camera, by adding additional calibration fields with skin tones and additional calibration fields providing a grey scale with smaller increments, have also been developed recently.
In a typical digital camera, there is a colour filter array, also called a colour filter mosaic, positioned in front of the pixel sensors of the camera's image sensor. The task of the colour filter array is to acquire colour information and it is necessary since the pixel sensors in general only can detect light intensity with little or no wavelength specificity. Thus, the pixel sensors cannot themselves separate colour information from the photographed subject. The colour filters in the colour filter array filter the light by wavelength range, so that the filtered intensities together provide information about the light colour. The most common type of colour filter array is so-called Bayer filters, which provide information about the light intensity in the red, green and blue wavelength regions. The raw data from the pixel sensors with overlying colour filter array can be converted to a full-colour image by using suitable algorithms.
When designing the previously known colour calibration cards no consideration has been given to the fact that the image sensor of a typical digital camera can only sense information about the light intensities in the red, green and blue wavelength regions and that the other colours are calculated based thereon. It has also been found that the colour filters for the respective three wavelength regions are not ideal in typical colour filter arrays, but asymmetrical, which results in a considerable transmittance also outside the transmittance peaks of the colour filters. Since the colours of the calibration fields on previously known colour calibration cards have primarily been selected to resemble the intended subjects, and without giving any consideration to the actual colour sensing system of a digital camera, a less reliable colour calibration result is achieved.